In printing devices, a media indexing system advances or indexes the media through the print zone during the printing process. Such systems can include motors, gears, rollers, sensors, and other components that work in conjunction to move the media incrementally. During each incremental movement of the media, a printing element, such as a print head, causes text and/or images to be applied to the media in rows or swaths.
However, manufacturing tolerances, assembly variations, and other imperfections in such indexing components can cause discrepancies between the desired or expected position of the media and the actual position which is realized. For example, one typical component in a media indexing system is a feedroll or feedroller which contacts the media and forces the media through the printing device. Manufacturing tolerances can cause the feedroller diameter to deviate from its ideal diameter and can also cause the feedroller circumference to deviate from its ideal circular shape. The deviation of the actual feedroller diameter from the ideal feedroller diameter can produce a cumulative feedrate error which corresponds to the average deviation in the desired media position during the movement. For example if the diameter is smaller than ideal, “runout” (feedrate) errors will occur causing the media to be underfed. In addition, a deviation of the actual feedroller shape from the ideal circular shape and/or a deviation of the axis of rotation from center, can produce irregular rotation and a substantially sinusoidal “once-per-revolution” (eccentricity) errors in the indexing of the media.
Other similar deviations in other indexing components can also introduce such errors in the indexing of the print receiving media. For example, eccentricity in the encoder wheel used to sense the feedroller position as well as various forces on the media being moved can cause the media to be advanced too little or too much. Such errors can vary within a printing device based upon the media type, environmental factors (such as temperature and humidity), and mechanism wear.
These media positioning errors can result in an undesirable appearance of the printed image. For example, indexing the media too far (i.e., overfeeding the media) might result in a gap or blank band between print swaths, while indexing the media too little (i.e., underfeeding the media) might result in overlapping print swaths that could create dark horizontal bands in the resulting image. Such swath misplacements can be particularly noticeable during particular printing modes or resolutions.
To compensate for such errors, media indexing calibration and compensation control systems and methods can be utilized. Such systems and methods can measure the system error and provide corresponding compensation to media indexing system components, to attempt to increase media positioning accuracy.
However, some media indexing calibration methods and systems can require expensive sensors and calibration sheets, as well as complicated calibration routines and lengthy correction tables, and therefore may only be suitable for use with expensive printing devices. Additionally, some such methods can require time-consuming and complicated calibration steps, such as those which require the acquisition of large amounts of data or the visual comparison of test patterns. Still other media indexing calibration techniques can suffer from accuracy problems.
Accordingly, it is desirable to provide improved media indexing calibration and error compensation methods and systems. In addition, it is desirable to provide improved systems and techniques for media indexing calibration that increase media indexing accuracy with greater efficiency and without high cost. Moreover, it is desirable to provide improved indexing calibration methods and systems that can accommodate error conditions.